1. Field of the Invention
The present invention relates to an apparatus and a method for control operating time of information display means, and more specifically to an apparatus for control display time of an IC card reader which measures the error of an oscillation frequency beforehand, and which varies a timer count number of times in accordance with the oscillation frequency error to compensate display time of an IC card reader.
2. Description of Related Art
The prior art will be described with regard to an IC card reader that is suitable for application of the present invention.
FIG. 7 shows the system configuration of an IC card reader. This is a system which use in combination with an IC card which serves as electronic money. In this IC card reader, an IC card 4 is inserted into a card reader/writer 3, a microcomputer 2 of the IC card reader reads the amount information of the money that has been deposited onto the IC card 4 by means of communications with the IC card, this amount information being displayed on an LCD panel 1.
The IC card reader is supplied to the user of an IC card as an accessory to the IC card at the time the IC card is purchased, and is a use-and-discard system that is discarded when the battery 5 thereof becomes depleted. For this reason, it is necessary to reduce the manufacturing cost as much as possible, and instead of using a high-cost quartz resonator as the resonator in the microcomputer system, an RC oscillator circuit, which has a low cost but also a low oscillation error accuracy, is used.
FIG. 8 is a block diagram which shows the clock circuit used in the IC card reader. The RC oscillator circuit 20 used for the main system clock has a CPU stop signal 36, which stops the oscillation, connected to it, so that when either a STOP signal 40, which is generated when the STOP command is executed, or the CPU stop register bit 41 is input, the CPU stop signal 36, which is produced by the OR gate 31, is input to the RC oscillator circuit 20 for the main system clock, thereby stopping the oscillation thereof.
In this microcomputer, to achieve an even further reduction in cost, the capacitor 33 in the RC oscillator for the main system clock, and the resistance 35 and capacitor 34 in the RC oscillator circuit 21 for the subsystem clock are built onto the silicon chip of the microcomputer.
The resistance 35 and the capacitors 33 and 34 that are built onto the silicon chip of the microcomputer exhibit variations in the manufacturing processes therefor, this resulting in a large error. With regard to RC oscillator circuit 21 for the subsystem clock in particular, since the resistance 35 and capacitor 34 are both built-in, depending upon the manufacturing lot, the oscillation frequency error can be as much as .+-.50%.
With regard to the main system clock, because a resistance 32 is mounted outboard with respect to the chip, enabling the use of an arbitrary resistance of high precision, the oscillation frequency error is relatively small, although in some microcomputers, in the case in which the main system clock 20 is used to operate the microcomputer, compared to the case in which the subsystem clock 21 operates the microcomputer, there can be an approximate 25-fold increase in power consumption.
It is necessary to have an IC card reader operate for a period of 2 to 3 years, and to extend the lifetime of the system batter 5, except at times when communication with the IC card 4 is being done, the main system clock 20 oscillation is stopped by inputting the CPU stop register bit 41.
For the reasons noted above, the control of the LCD display time is performed by the subsystem clock 21 operating a timer. Therefore the oscillation frequency error is large and, there is a large variation in the LCD display time.
However, because an IC card system is for the purpose of receiving the information of the amount of money deposited onto an IC card and displaying this information on an LCD display panel, the LCD display time accuracy is a very important factor, and it is generally necessary to hold the display time error to within about .+-.20%.
For example, if the LCD display time is shorter than an established time, whereas the display time for one screen requires 2 seconds, the screen will, for example, be displayed for only approximately 1.3 seconds (in the case in which the oscillation frequency error is .+-.50%), making the displayed information difficult to see.
In the reverse case, if the LCD display time is excessively long, for example, in the case in which the oscillation frequency error is -50%, a simple calculation shows that the current consumption will be increased to a maximum of 1.5 times, thereby resulting in a commensurate shortening of the battery life.
Next the display time control method in which the above-noted problem occurs will be described. FIG. 9 is a block diagram of the time control system used in the prior art. In the display time control system used in the past, the number of the timer interrupt request signal is set to the display time setting program, and the timer that is operated by the subsystem clock is started. Then, a number of timer interrupt request signals equal to the number of the timer interrupt request signal which is set to the display time setting program and, at the point at which the count is completed, the display is ended.
FIG. 10 (a) and (b) are flowcharts of the time control system of the past. First, the display time setting procedure shown in FIG. 10 (a) will be described. The number of the timer interrupt request signal is set (170), and the timer that is operated by the subsystem clock is started (171).
Next, the display time control procedure shown in FIG. 10 (b) will be described. In this procedure, the timer interrupt request signal is monitored (180) and, if an interrupt request is generated, the number of interrupt request signal is decremented (181). Then, the number of the timer interrupt request signal is checked to see if it is 0 and, if it is 0 (182), the display is ended (183). As shown in FIG. 11, in the case in which the timer interrupt request signal generates every 500 ms and the display time is 2000 ms, the display time is controlled by counting the timer interrupt request signal 4 times.
Next, the case in which an error occurs in the subsystem clock of the microcomputer will be described. In the case in which an error in a constituent element causes the oscillation frequency of the subsystem clock to have an error on the excessively high side, so that, as shown in FIG. 12, if the timer interrupt request signal generates every 370 ms (an oscillation frequency error of .+-.35%), when the timer interrupt request signal is counted 4 times, the display time is 1480 ms.
In the reverse condition, in which the oscillation frequency is excessively low, as shown in FIG. 13, if the timer interrupt request generates every 770 ms (an oscillation frequency error of -35%), when the timer interrupt request signal is counted 4 times, the display time is 3080 ms.
In this manner, when using a microcomputer into which are built a resistance and a capacitor for use in a system clock, error in the capacitance and resistance values that make up the resonator cause a frequency error in the system clock, thereby preventing the accurate operation of the display time control timer, this resulting in variations in the display time. When these variations occur, the readability of the displayed information declines, and the power consumption increases.
In this case, in a microcomputer which has only a timer having a long time from the timer start until the generation of a timer interrupt request, when the above-noted timer is used to control the display time, even if the number of counts of the timer interrupt request signal is changed, it is not possible to compensate the error in a display time.
In view of the above-described drawbacks in the prior art, an object of the present invention is to compensate the display time error and improve performance, by varying the number of counts of the timer interrupt request signal.